The present invention is directed to an improved gas laser apparatus, method of lasing gas and a turbine type compressor therefor. More particularly, the invention relates to an improved fast axial flow gas laser apparatus wherein gases are circulated in a closed loop through the laser tube at speeds approaching the speed of sound in the laser gas.
In known fast axial flow gas lasers the gas is generally moved through the laser by use of a Roots blower. A Roots blower is characterized by the use of two rotors which are moved in synchronism in relation to one another by means of gears and a driving motor. Typically, the rotors rotate at a speed of 3,600 rpm. A gas laser with a single stage Roots blower or pump operating at this speed is disadvantageous for certain laser applications because the output of the blower has a 240 Hertz fluctuation in its discharge pressure. This pulsing or reverberation in the pressure output of the blower results in a corresponding fluctuation or instability in the laser discharge which, in turn, causes instability in the power output of the laser. Such fluctuations in the power output of the laser are unacceptable in many applications, e.g., fine engraving, cutting, welding, etc. If the discharge flow of the Roots blower is dampened to avoid these pressure fluctuations, there is a reduction in the useful output power or efficiency of the blower and the associated gas laser. The vibration also associated with the Roots blower can be disadvantageously transmitted to the laser itself to actually vibrate the output laser beam.
The gears and bearings normally employed in the typical Roots blower require lubrication. The grease or oil used for this purpose poses a serious problem for the operation of both the blower and the associated laser. That is, if this oil leaks into the pumping chamber of the Roots blower, it can break down and be deposited on the lobes of the blower impellers, closing the small gaps between the lobes and adjacent pump housing and causing the lobes to seize in the pump housing. Oil leaked into the laser gas will be vaporized in the laser tube thereby effecting discharge stability. Oil vapor deposited on the optical elements of the laser results in deterioration of the laser performance and reduces the life of the optics. To prevent these occurrences, vacuum chambers are provided in Roots blowers adjacent the pumping chamber. A higher vacuum is maintained in the vacuum chambers than in the pumping chamber, so that any oil leaking from the gears and bearings will be drawn into these higher vacuum chambers and not the pumping chamber to thereby maintain the integrity of the blower and laser. The provision of such protective, higher vacuum chambers adds to the cost of the blower and contamination can still occur in the event of failure of the higher vacuum chamber.
The Roots blower is also disadvantageous because of its considerable size and weight. Further, it requires frequent servicing of lip seals provided therein about the rotary shaft of the blower. Every 800 to 1,000 hours of operation a trained technician must shut down the operation of the blower to service the lip seals. The down time associated with this type of service as well as the labor costs of the trained technician increase the cost of the related manufacturing operation.
An object of the present invention is to provide an improved gas laser apparatus, method of lasing gas and a turbine type compressor therefor which avoid the aforementioned disadvantages of known gas laser apparatus and methods employing the conventional Roots blower. More particularly, an object of the present invention is to provide a gas laser apparatus which has a continuous, stable discharge and power output, so that fine engraving, cutting, welding, etc., can be performed.
A further object of the invention is to provide an improved gas laser apparatus, method of lasing gas and a turbine type compressor therefor wherein the problems of lubricant contamination of the laser gas in the compressor are eliminated without the provision of special higher vacuum chambers adjacent the compressor.
An additional object of the invention is to provide an improved gas laser apparatus, method of lasing gas and a turbine type compressor therefor, wherein the size and weight of the compressor are reduced as compared with the conventional Roots blower and wherein the compressor has a long life and does not require the frequent servicing of lip seals required by a Roots blower.
These and other objects of the invention are attained by the gas laser apparatus of the invention which comprises means defining a flow path for a laser gas, means for exciting gas flowing in the apparatus to cause the gas to lase, and a turbine type compressor for flowing gas along the flow path, wherein the compressor has a head coefficient of at least 0.8 and is capable of operating with a pressure ratio sufficient to flow the gas along at least a portion of the flow path at a speed of at least half the speed of sound in the laser gas with an inlet pressure to the compressor of less than one-third atmospheric pressure. The means defining the flow path forms an at least essentially closed loop for recirculating gas through the laser apparatus. The closed loop includes the compressor.
The specific pressure ratio pr of the compressor depends on the gas or mixture of gases used with the laser apparatus and the mixing ratio of the components of the gas mixture. With a gas mixture of helium, nitrogen and carbon dioxide according to a disclosed embodiment, the compressor is capable of operating with a pressure ratio of at least 1.5:1 with an inlet pressure of between 50 and 100 torr, for example, and a mass flow through the compressor on the order of several hundred cubic feet per minute or more.
In the disclosed, preferred embodiment of the invention, the turbine type compressor for flowing gas through the laser tube is a regenerative compressor which comprises an impeller rotatable about an axis and having generally radial blades extending from at or near the tip of the impeller inward a distance of no more than about 50% of the radius of the impeller. Axially opposite the blades of the impeller is a means defining a stationary annular passage with an inlet and an outlet being provided for communicating the gas to and from the passage for peripheral flow in the passage in the direction of rotation of the impeller. A dam is provided blocking the annular passage between the inlet and outlet. The dam has close clearance over the impeller. In the disclosed embodiment, the regenerative compressor is a two stage compressor with the respective stages being located on opposite sides of a single impeller. An intercooler is provided between the first and second stages of the compressor for cooling gas compressed in the first stage before it enters the second stage for further compression. The intercooler includes a heat exchanger formed with a plurality of concentric tubes where the passages between adjacent tubes respectively convey the gas and a coolant fluid for heat exchange to cool the gas as it passes through the heat exchanger. Both sides of the compressor housing and also the radially outer or circumferential surface thereof are also cooled by circulating coolant through passages provided therein so that the gas is cooled as it is being compressed. This is particularly important for effective cooling and efficient compressor operation especially with the very low pressures at which the compressor operates. Approximately 50% of the required gas cooling to remove the heat of compression is taken out by the cooling incorporated in the compressor housing. This gives the effect of interstage cooling which results in increased compression efficiency.
The impeller of the regenerative compressor is rotatably supported on a drive shaft of the compressor at a first location along the shaft. The impeller is rotated about the longitudinal axis of the drive shaft at a high speed such that the circumferential speed of the impeller is a substantial fraction of or near the sonic speed for the laser gas. In this way the acceleration of the gas in the compressor approaches the speed of sound in the gas in the compressor at the tip of the impeller blades thereby minimizing friction losses. In the disclosed embodiment, the speed of rotation is about 10,000 rpm.
Lubricated bearing means rotatably support the drive shaft at at least a second location along the shaft spaced from the first location. A positive pressure fluid seal means is provided for preventing lubricant from the bearing means from moving along the drive shaft to the impeller and contaminating the laser gas. The fluid seal means includes a mating ring sealingly attached to the drive shaft between the first and second locations for rotation with the shaft, a pair of annular, spaced stationary members located about the shaft and presenting respective sliding faces for contacting respective opposite sides of the mating ring, and means for directing a fluid or buffer gas under a pressure slightly above the pressure of the gas in the compressor, between the sliding faces and the mating ring and along the drive shaft during operation of the compressor to prevent lubricant from the bearing means from moving along the drive shaft to the impeller. In the preferred embodiment of the invention, the buffer gas, which is non-contaminating with respect to the laser gas, is permitted to migrate into the laser gas to prevent leakage of atmospheric gas into the laser gas and to serve as a make-up gas for losses of the laser gas. Preferably, the buffer gas is the same kind of gas used in the laser.
The drive shaft and the impeller in the disclosed embodiment of the compressor are rotated at a speed of about 10,000 rpm and because in each stage of the compressor the impeller carries characteristically 30 blades thereon, the pressure of the compressor discharge is continuous and stable. Therefore, a more continuous, stable laser discharge and laser power output can be produced. The size and weight of the regenerative compressor of the invention are also less than those of the typical Roots blower which permit a reduction in the size and weight of the gas laser apparatus. The regenerative compressor also has a relatively long life and needs only infrequent servicing. Gas, magnetic, ball or roller bearings can be used in the compressor without fear of contamination of the laser because of the special sealing arrangement of the invention. Further, the requirements for two rotors and gears as in Roots blowers are avoided with the regenerative compressor of the invention.
The method of lasing gas in a fast axial flow gas laser according to the invention comprises the steps of compressing a gas in a turbine type compressor operating with a head coefficient of at least 0.8 and with a pressure ratio sufficient to flow the gas along at least a portion of a flow path for the gas in the laser at a speed of at least half the speed of sound in the gas, conveying gas compressed by the compressor along the flow path for said gas in the laser and exciting said gas to cause it to lase. The gas is recirculated through the laser in a closed loop. The compressor forms part of the closed loop flow path for the gas. Preferably the gas is moved at speeds which approach or even exceed the speed of sound along at least a portion of the flow path in the laser. The method further includes the steps of cooling the gas both before, during and after compression in the regenerative compressor as it moves through the closed loop of the apparatus, and positively sealing the lubricant in the compressor against movement into the laser gas by means of a pressurized fluid seal. The pressurized fluid of the seal is permitted to move into the gas being compressed to make-up for lost laser gas.
These and other objects, features and advantages of the invention will become more apparent from the following description when taken in connection with the accompanying drawings which show, for purposes of illustration only, several embodiments according to the invention.